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period data available in IMD for the period of 1891-2011. We have studied it
for different intensities such as D and above, CS and above, SCS and above
with respect to the whole Indian region including NIO by considering annual
as well as season-wise frequencies for pre-monsoon, monsoon and post-
monsoon seasons at inter-decadal time scale using data available from IMD
for 1891-2011. We have computed anomalies from long-term means for
respective frequencies to find amplitude and epochal periodicity of total intense
cyclonic systems over Indian region, BoB and AS from 11-years running means
for 121 years (1891-2011) of D and above, SCS and above for whole Indian
region, BoB and AS. We have also studied contribution of three major cyclonic
seasons e.g. pre-monsoon, monsoon and post-monsoon to total annual
frequencies in terms of % of total system formed in each of these major three
cyclonic seasons to the total systems of D and above and SCS and above
formed in each year for whole period of 121 years and identified climatic shift
if any in their seasonal contribution at decadal time scale.
For studying decadal changes if any in basin-wise intensification or
intensification over Indian region as a whole and their associated climatic shifts,
we have computed Intensification ratio (IR
)
of cyclonic system over Indian
region, AS, BoB defined as ratio of total SCS intensified in a year or a season
to total number of systems of depressions and above formed in that year for
respective region. This has been studied both for annual and for cyclone seasons
of pre-monsoon and post-monsoon. AS normally has very lower number of
cyclonic systems compared to their very higher frequencies over BoB. In view
of % of number of D and above formed over AS during recent years including
at intensities of SCS and above to their annual total have been higher while for
BoB, it has been in decreasing trend or stagnated, we have studied how
cyclogenesis ratio of AS to BoB at respective intensities has changed over
the period.
2. Data and Methodology
In the present study, we have collected monthly frequencies of cyclonic systems
of intensities of D and above, CS and above, SCS and above from IMD for the
Indian region for the period 1891-2011. We have analyzed them with respect
to their total annual frequencies and then frequencies for major three seasons
covering pre-monsoon (March-May), monsoon (June-Sept) and post-Monsoon
(Oct-Dec) through both linear trend and the low-pass filtered (applying 11-
year running mean) time series which highlight the multi-decadal variations.
We have also analyzed them by separating them into frequencies over AS and
BoB region separately available since 1891 to 2011 with IMD vis-a-vis their
annual and seasonal frequencies.
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